Preparation of N-substituted aspartic acids via enantiospecific conjugate addition of N-nucleophiles to fumaric acids using methylaspartase: synthetic utility and mechanistic implications1

Abstract

A range of new N-substituted aspartic and 3-alkylaspartic acids are prepared via the enantiospecific conjugate addition of substituted amines to fumaric acids using the enzyme methylaspartase. The stereochemical courses of the additions to mesaconic acid are determined for hydrazine and hydroxylamine. Each addition is found to follow the same course as that for ammonia, the natural nucleophile for the enzyme; anti-addition to the 3-si-face of mesaconic acid gives the (2S,3S)-3-methylaspartic acid derivative. The addition of hydrazine, methylamine, hydroxylamine and methoxylamine occurs in excellent conversion and the resulting hydrazino- and methylamino-succinic acids can be isolated for full characterisation. The corresponding hydroxyamino- and methoxyamino- succinic acids are recovered as unstable oils. The size of the substituent on the N-nucleophile tolerated by the enzyme displays a profound dependence on the size of the substituent on the acceptor fumaric acid in a mutually exclusive manner. This finding indicates that the two substituents are able to access close regions in three-dimensional space at the active site of the enzyme.